Before we start, I want to thank Whites Thinking Outside for sending the Retain-it posts. Greatly appreciated! Check out their Facebook page for garden and outdoor ideas. And here’s my previous DIY project using Whites Screen Up posts.

So, there are a few different ways of building a planter box but for this one, we will be using timber sleepers and Whites Retain-it steel upright posts. These materials are normally for retaining walls so for this project, we will be doing both — a retaining wall and a planter box.

You may have seen this around, mostly in galvanised steel posts and rough-sawn timber. With Whites’ new black upright posts, some stain and varnish, we will try to take the aesthetics up a notch.

Now let’s get down to business!

The Problem

So we have this small backyard, around 3m x 10m of which is carpeted with a Sir Walter Buffalo grass. As much as wish I had a bigger backyard, the relatively smaller size actually makes it a lot easier to manage, and for that it’s perfect. There’s one little problem though — it has a bit of a slope. As a result, my soil gets slowly washed away by heavy rains. Also, I didn’t want to plant directly on the ground without something to shield my fence. That means, having new plants would require us to buy new pots as well.

The Solution

A retaining wall and a planter box. Retaining wall for the erosion problem, planter box for the missus’ plants. Everybody happy.

The Plan

The timber retaining wall will run along the back fence. This will be buried at least 200mm (1 sleeper) below to stop further soil erosion.

The planter box will replace our existing potted-plant corner. This way we won’t have to buy pots every time we get new plants. They will be planted straight into the planters and I won’t have to worry about my fence.

As a plus, the planter box will be capped with a ledge — perfect seating for a fine BBQ & beer day (talk about ulterior motives).

What You’ll Need

There are only 3 key materials needed, simple as that. All the others are just blings, if you may. And before you buy your materials, be sure to take measurements and come up with a plan.

  1. Whites Retain-it Posts
whites retain it posts
steel upright posts
sleeper post

This comes in 3 different types: End posts, Corner posts, and Joiner Posts. I used all types for this project.

I also used varying lengths – 1100mm (3 sleepers high) and 750mm (2 sleepers high).

Depending on how big and how complex your design is, you’ll have to spend some time planning. Draw if you can.

2. Treated Pine Sleepers

Timber sleepers
Treated Pine
Treated pine sleepers

Make sure it’s treated pine H4, suitable for outdoor use.

I used 50mm thick treated pine sleepers.

The sleepers come in 1.8m, 2.4m and 3m lengths. I personally think 3m is too long – could be more prone to bending/warping. The closer the posts, the better.

3. Fast-setting Concrete

fast setting concrete

The normal concrete would be fine but if you have a relatively big project, you’d want to use the fast-setting one.


Since you already have your materials, I will assume that you have done your measurements, you’ve planned your layout, and have done your estimates.

Time to get our hands dirty. Literally!

Dig the 1st hole

post hole digger
post hole
Use a posthole digger and leave enough height for 2 sleepers (40cm) above ground.

Put the first Retain-it post in position

retain it end post
Don’t pour concrete yet, we can’t afford to make mistakes.

Dig a trench

From the first post up until about the length of the timber (2.4m for mine).

Put the timber in position

treated pine sleeper
This is just to know exactly where to dig next. I don’t trust my tape skills.

Dig the 2nd hole

post hole digger
This is your workout for the day. Dig dig dig!

Pour concrete on the first hole

spirit level
Use a spirit level and make slight adjustments while the concrete hasn’t settled.

Level the 2 posts

using a spirit level
In hindsight, I should have just left the sleeper in position. *facepalm*

Slide the 1st sleeper in and pour concrete on the 2nd post

whites steel end post
whites joiner post
sleeper post
Use a spirit level on the post and sleeper. Make necessary adjustments.

Slide in the next sleeper

spirit level
using a spirit level
And again, check the level. Just because it looks cooler with the spirit level on.


That’s about it. You have all the skills now, you can build anything — a bridge, a tunnel, Great wall of China, you name it!

At this point, it’s just a matter of repeating all those steps until you are done. Much like copy-paste — as a former software developer and currently, a BA, I was born with it, moulded by it! πŸ˜€

Handy Tip: The Retain-it posts can hold sleepers at 30-degree angles. That means, instead of having a 90-degree corner, you can actually make it appear like a curve/arch.

Whites retain it joiner post
Joiner post angle
30-degree angles on both sides.
Whites retain it joiner post angle
A series of 30-degree angles giving that “curved” corner look.

Actually, we still have a few things to do, but they’re optional. What we have already done so far is a fully functional retaining wall and planter box.

Put that beer back in the fridge!

Let’s Cap it Off

The ledge or capping is another story and it has to be the last. It requires a little bit of finesse, some attention to detail, angles (Trigo! MATH!). In my case, that curved section only meant more angles to cut with lightsaber-like precision. Don’t worry, I got away without having to use a protractor. My Trigonometry teacher will be disappointed.

Okay, back to work!

Measure and cut the timber

circular saw
Using circular saw on a miter cut. Best way to do this is with a miter saw, which I don’t have.

Pre-drill with a countersink drill

countersink drill
flush finish
I used a #14 counter sink to match my 14G bugle head screws


flush finish screw bugle head screw
This is a self-drilling screw but we needed the pre-drill for this flush finish.


random orbital sander
ozito sander
I did this for the whole thing, not just the ledge. And please don’t work barefoot, safety 1st. πŸ™‚


merbau externa water-based stain
I used water-based external stain. Dries quicker than oil-based.

DONE! Pop that beer open.

Helpful Tips

  • A rubber mallet is your best friend when levelling. Well, next to the spirit level of course. Instead of digging a few mm, just hammer through the timber. Very useful for small level tweaks.
  • For sleepers at 2m in length or more, I put timber posts in the middle for reinforcement. They still feel strong and robust without it but we don’t want to take chances
  • If you have time, use an oil-based stain. In my experience with both oil and water-based, the oil-based ones tend to be easier to work with, and leaves me with a more even finish. Takes too long to dry though, say 24 hours.
  • Always be mindful of your posture when lifting (you’ll be doing this a lot) these sleepers. You don’t want to hurt your back just for this project, totally not worth it.


Here are some before and after photos.

Swipe sideways
Swipe sideways
Aquapro 185L pond 
Tiny pond
Patio pond
If you’re wondering, that’s a pond. And that’s another How To story. πŸ™‚

The missus wanted me to publish this once she’s done planting. She’s probably right, plants make a huge difference and would definitely inspire you more. I cannot wait that long though, the world needs my DIY guide! πŸ˜€

What more can I say, give this project a shot. Something this size is not really a walk in the park but it’s so fulfilling in the end.

Like everything we do in life, there will be challenges along the way, but just bite the bullet and don’t forget to have fun.

See you on the next build!